Hook Functions in the Apache HTTP Server 2.x

Warning

This document is still in development and may be partially out of
date.

In general, a hook function is one that the Apache HTTP Server
will call at some point during the processing of a request.
Modules can provide functions that are called, and specify when
they get called in comparison to other modules.

Use the AP_DECLARE_HOOK macro, which needs to be given
the return type of the hook function, the name of the hook, and the
arguments. For example, if the hook returns an int and
takes a request_rec * and an int and is
called do_something, then declare it like this:

AP_DECLARE_HOOK(int, do_something, (request_rec *r, int n))

This should go in a header which modules will include if
they want to use the hook.

The source file that exports the hook has to implement a
function that will call the hook. There are currently three
possible ways to do this. In all cases, the calling function is
called ap_run_hookname().

Void hooks

If the return value of a hook is void, then all the
hooks are called, and the caller is implemented like this:

AP_IMPLEMENT_HOOK_VOID(do_something, (request_rec *r, int n), (r, n))

The second and third arguments are the dummy argument
declaration and the dummy arguments as they will be used when
calling the hook. In other words, this macro expands to
something like this:

The first hook that does not return DECLINED
stops the loop and its return value is returned from the hook
caller. Note that DECLINED is the traditional
hook return value meaning "I didn't do anything", but it can be
whatever suits you.

Alternatively, all hooks can be run until an error occurs.
This boils down to permitting two return values, one of
which means "I did something, and it was OK" and the other
meaning "I did nothing". The first function that returns a
value other than one of those two stops the loop, and its
return is the return value. Declare these like so:

In the example above, we didn't use the three arguments in
the hook registration function that control calling order.
There are two mechanisms for doing this. The first, rather
crude, method, allows us to specify roughly where the hook is
run relative to other modules. The final argument control this.
There are three possible values: APR_HOOK_FIRST,
APR_HOOK_MIDDLE and APR_HOOK_LAST.

All modules using any particular value may be run in any
order relative to each other, but, of course, all modules using
APR_HOOK_FIRST will be run before APR_HOOK_MIDDLE
which are before APR_HOOK_LAST. Modules that don't care
when they are run should use APR_HOOK_MIDDLE. These
values are spaced out, so that positions like APR_HOOK_FIRST-2
are possible to hook slightly earlier than other functions.

Note that there are two more values,
APR_HOOK_REALLY_FIRST and APR_HOOK_REALLY_LAST. These
should only be used by the hook exporter.

The other method allows finer control. When a module knows
that it must be run before (or after) some other modules, it
can specify them by name. The second (third) argument is a
NULL-terminated array of strings consisting of the names of
modules that must be run before (after) the current module. For
example, suppose we want "mod_xyz.c" and "mod_abc.c" to run
before we do, then we'd hook as follows:

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